The present invention relates to accommodating loading and, more particularly, to apparatus for tailoring the response to such loading applied by reason of motion of one structure relative to another. In a preferred implementation, it relates to a passive tensioner for absorbing the loading caused by relative motion between a floating oil or other platform and a riser or the like which has either no, or a defined, motion relative to wave or other liquid action on the platform, and a method for absorbing such loading.
It will be recognized that floating or semisubmerged oil platforms are subjected to relatively constant movement due to wave motion, current drift or other liquid action. These platforms, though, have one or more riser strings which extend into the sea floor for any of various purposes relating to the exploration for, or the extraction of, a liquid fossil fuel such as crude oil. These riser strings do not, of course, move with the typical liquid action.
Tensioners have been developed in the past for generating tensile loads to accommodate the loading generated between a floating platform and riser strings because of the relative motion or forces between the same. In this connection, tension is applied to the top of the riser string between an adapter that attaches to the uppermost riser and the platform foundation. That is, a tailored tension is maintained on the riser strings to prevent failure due to buckling or bending.
The majority of tensioners developed and used in the past have been active devices. Either a pressurized gas or fluid is used in the cylinder to maintain the desired tension force while compensating for any motion of the platform. It has been believed that an active tensioner is necessary to provide the tailored response to the motion which is desired. However, passive tensioners also have been designed in the past. An example is that described in U.S. Pat. No. 5,101,905 issued Apr. 7, 1992 to Arlt et al. In general, though, such tensioners have not been designed to allow one to tailor the response to the loading, based on the displacement it might represent. In this connection, while it is noted in the Arlt et al patent that one can vary a spring force and achieve differing dynamic characteristics by using differing configurations of elastomeric spring elements (see Arlt et al col. 3, lines 58-61), Arlt et al certainly does not describe how such might be done and what criteria is important to do so.